Interest in catalysis continues to grow in the polyolefin industry. White much attention has been paid to single-site catalysts (metallocene and non-metallocene), Ziegler-Natta catalysts are a mainstay for polyolefin manufacture. Much research has been done since their inception and there are many types of Ziegler-Natta catalysts in use.
There are many known processes to polymerize olefins. Most are solution, slurry, or gas-phase processes. Solution processes are described, e.g., in U.S. Pat. Nos. 4,311,816, 4,769,428, 5,492,876, 5,589,555, 6,127,484, and 6,221,985. Solution processes operate at temperatures above the melting or solubilization temperature of the polymer.
While there are advantages of solution polymerizations, one disadvantage is that the high temperatures needed can cause catalyst decay. Because of this instability, the typical catalyst has poor activity and a high level is needed for good polymerization rates. Unfortunately, leaving high levels of residual catalyst in the polymer can adversely affect physical and mechanical properties, including ultraviolet stability. One approach is to deactivate or remove the catalyst, but this is costly.
One partially successful way to improve activity is to use mixed catalysts. For example, U.S. Pat. Nos. 3,218,266, 4,483,938, 4,739,022, and 5,492,876 use mixtures of vanadium and titanium-based Ziegler-Natta catalysts. While there is an improvement in activity, the levels of residual catalyst are too high for many end-use applications without deactivation or catalyst removal.
There have been some instances of linking three metals. U.S. Pat. Nos. 4,324,736 and 4,387,199 describe tetravalent vanadium compounds containing two titanium atoms with general formula:

The vanadate(IV) esters are said to have good solubility and improved stability versus vanadium alcoholates, making them suitable as catalysts for polymerizing olefins, dienes, and mixtures thereof. However, no polymerization information is given for these trimetallic esters. There is also no indication that a bimetallic catalyst could be used. A trimetallic catalyst requires three moles of transition metal per mole of active catalyst, thereby increasing the amount of transition metal in the polyolefin.
Trimetallic vanadates having the general formula:
have been used for suspension polymerization of ethylene at 85° C. (see Herrmann et al. Makromol. Chem. 94 (1981) 91). Many catalyst systems are effective for suspension polymerizations but are ineffective for solution polymerizations.
A complex with two metals linked with an aliphatic diol is disclosed in German Patent DE 1,254,638. The complex is identified as a catalyst component, but the reference gives no polymerization information. Because there is no opportunity for conjugation through the aliphatic diol, it is unlikely that the catalyst would have special stability when used in a high-temperature polymerization.
Bimetallic compounds are known. For instance, Chem. Mater. 10 (1998) 620 discloses the synthesis of a vanadium-titanium alkoxide on a silica surface by means of sequential chemical vapor deposition, but there is no indication that the product might be used for polymerizing olefins. Bimetallic systems with an oxo linkage have been reported in J. Am. Chem. Soc. 117 (1995) 2210 and in J. Am. Chem. Soc. 118 (1996)10175. Again, neither of these indicates that the bimetallic compound is useful for polymerizing olefins.
There remains a need for a solution process with improved catalyst activity. If the catalyst activity is improved, the cost of the deactivation and removal steps can be decreased or even eliminated for some end-use applications.